Pretreatment technologies for anaerobic digestion of lignocelluloses and toxic feedstocks

Journal Pre-proofs Review Pretreatment technologies for anaerobic digestion of lignocelluloses and toxic feedstocks Ria Millati, Rachma Wikandari, Teguh Ariyanto, Rininta Utami Putri, Mohammad J. Taherzadeh PII: DOI: Reference:

S0960-8524(20)30267-4 https://doi.org/10.1016/j.biortech.2020.122998 BITE 122998

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Bioresource Technology

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29 October 2019 4 February 2020 8 February 2020

Please cite this article as: Millati, R., Wikandari, R., Ariyanto, T., Putri, R.U., Taherzadeh, M.J., Pretreatment technologies for anaerobic digestion of lignocelluloses and toxic feedstocks, Bioresource Technology (2020), doi: https://doi.org/10.1016/j.biortech.2020.122998

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Pretreatment Technologies for Anaerobic Digestion of Lignocelluloses and Toxic Feedstocks Ria Millatia,*, Rachma Wikandaria, Teguh Ariyanto b, Rininta Utami Putria, Mohammad J. Taherzadehc a

Department of Food and Agricultural Product Technology, Universitas Gadjah Mada, Yogyakarta

55281, Indonesia b

Department of Chemical Engineering, Faculty of Engineering, Universitas Gadjah Mada, Jl Grafika

No 2, Yogyakarta 55281, Indonesia c

Swedish Centre for Resource Recovery, University of Borås, 50190 Borås, Sweden

*Corresponding author. Tel./Fax: +62-274-549650. E-mail address: [email protected] Abstract Several feedstocks for anaerobic digestion (AD) have challenges that hamper the success of AD with their low accessible surface area, biomass recalcitrance, and the presence of natural inhibitors. This paper presents different types of pretreatment to address those individual challenges and how they contribute to facilitate AD. Organosolv and ionic liquid pretreatments are effective to remove lignin without a significant defect on lignin structures. To deal with accessible surface area and crystallinity, comminution, steam explosion, pretreatment using N-methyl-morpholine-N-oxide methods are suggested. Moreover, solid extraction, simple aeration, and biological treatments are capable in removing natural inhibitors. Up to date, methods like comminution, thermal process, and grinding are more preferable to be scaled-up. Keywords: Anaerobic digestion; Pretreatment; Lignocelluloses; Toxic feedstocks

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1. Introduction Anaerobic digestion (AD) is an old yet promising technology for handling various waste materials with high moisture content and impurities in large and small scales (Appels et al., 2011). The wide range of feedstocks starts from easily digestible to hard-to-digest materials from various sources such as agricultural sectors, forest residues, industrial waste streams, and municipalities. Anaerobic digestion poses several benefits to the environment such as reduction of air and water pollution, and replacement of inorganic fertilizer (Panigrahi and Dubey, 2019). In addition, AD is used traditionally to produce biogas, and also recently for the production of hydrogen and/or volatile fatty acids (Wainaina et al., 2019). Hydrogen is a chemical that is used as an energy source and a material that is used for chemical industries, whereas volatile fatty acids are intermediate compounds to produce a wide variety of products (Wainaina et al., 2019). Among the products that can be generated via AD, biogas is the frequent end product as it holds many applications such as heating, cooking, electricity, and car fuel. The successful development of AD process is affected by the characteristic of feedstocks used. The natural defense of plants including physical and chemical barriers often becomes the challenges during biomass degradation. For hard-to-digest feedstocks, the challenge comes from the physical barrier in the form of structural complexity, which makes biomass difficult to be accessed by digesting microorganisms. For easily degradable feedstocks, the challenge comes from the chemical barrier caused by the presence of bioactive compounds that have antimicrobial activity (natural inhibitors). Hence, the presence of both physical and chemical barriers can reduce the efficiency of AD process. The recalcitrant of biomass makes its natural decompositions occur very slowly and even take months to years, while the industrial application of biomass for biofuels must be done in a matter of days (Balan, 2014). In addition, biogas production ceased when antimicrobial compounds in citrus waste, i.e. D-Limonene, presents at 400-900 mg/kg (Mizuki et al., 1990; Forgács, 2012). Considering these

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facts, a pretreatment process in order to deal with physical and chemical barriers that leads to improved biogas production is of importance. Several review papers on pretreatment have been published (Amin et al., 2017; Kainthola et al., 2019; Patinvoh et al., 2017; Raj et al., 2017; Taherzadeh and Karimi, 2008; Zheng et al., 2014). However, most of them discuss pretreatment based on the classification methods i.e. physical, chemical and biological pretreatments. In addition, scarce information is available on pretreatment for inhibitor containing feedstocks. Therefore, the aim of this work is to use a different perspective by discussing the development and mechanism of pretreatments to deal with individual challenges faced by hardto-digest materials and feedstocks with natural inhibitors. Furthermore, the status of commercialization and techno-economical aspect of different pretreatment are addressed. 2. The challenges in biogas production with regards to feedstock Physical barrier is commonly found in lignocellulosic biomasses and keratin-based materials. Lignocelluloses consist of cellulose, hemicellulose, and lignin. Cellulose is a linear polymer of β-1,4 glucan, whereas hemicellulose is a polymer of various sugars and uronic acids. Cellulose chains do not stand alone. They tend to form intra and intermolecular hydrogen bonds through hydroxyl groups, which produce a supramolecular structure with high degree of polymerization (DP) called microfibrils (Himmel et al., 2007). The strong interchain hydrogen bonds create crystallinity of cellulose, which make it difficult to be digested during enzymatic hydrolysis. Besides crystallinity, the barrier comes from inter linkage between cellulose and hemicellulose, and further cementation by lignin through covalent and non-covalent bonds, and create a complex and recalcitrance lignocellulosic matrix (Zhang et al., 2019). In summary, biomass recalcitrance is affected by several factors i.e. lignin barrier, accessible surface area, and cellulose crystallinity. Lignin requires high temperature (e.g. 180 °C) and different acidity to be dissolved (Grabber, 2005), for which reasons lignin is regarded as the most recalcitrant component of plant’s cell wall. It has been widely accepted that the higher the lignin content, the greater the resistance of biomass towards 3

degradation. Besides the concentration, the type of lignin also affects biomass recalcitrance. Guaiacyl lignin is reported to have ability to prevent fiber swelling and enzyme accessibility more than syringyl lignin (Ramos et al., 1992). Acessible surface area becomes a challenge in the hydrolysis of lignocellulose because cellulose is often covered by other components, which limit its direct physical contact with cellulolytic enzyme. The accessible surface area also correlates with the pore volume and crystallinity (Taherzadeh and Karimi, 2008). There are two types of surface areas i.e. (i) external surface, which is correlated to the size and the shape of particle and (ii) internal surface, which depends on the capillary structure of cellulosic fibers. Furthermore, crystallinity is responsible to the hardness of a material, CI is then associated with the resistance of material towards biological degradation. The higher the CI, biodegradation of cellulose becomes more difficult. It is therefore widely accepted that decreasing CI might contribute to increase the rate of lignocellulosic digestion. The second challenge is the chemical barrier, which is present in the form of natural toxicants of the plant. Several studies reported some natural toxicants for anaerobic digesting microbia such as Dlimonene in citrus (e.g. Forgács, 2012; Wikandari et al., 2013); patchouli oil in patchouli leaves (Safarudin et al., 2018); mrycene, car-3-ene, α-pinene, and octanol in orange, mango, plum, strawberry, and grape (Wikandari et al., 2013); as well as tannins and saphonine in magosteen peel (Sanjaya et al, 2016). According aforementioned studies, these toxicants negatively affect anaerobic digestion. For instance, anaerobic digestion of citrus waste containing D-limonene resulted in 12% of methane yield from its theoretical value (Wikandari et al., 2014). Furthermore, no biogas could be produced from mangosteen peel (Sanjaya et al, 2016). As different feedstocks have different characteristics and challenges, hence the goal of pretreatment depends on the feedstocks as shown in Fig. 1. For lignocelluloses, a pretreatment process aims to open up biomass recalcitrance to increase its biodegradability. For inhibitor-containing feedstocks, the pretreatment goal is to eliminate the inhibiting compounds. Therefore, it is important to consider several factors to make pretreatment efficient and economically attractive, i.e. (i) avoiding the loss of 4

carbohydrate or the destruction of cellulose, hemicellulose, and lignin; (ii) avoiding the formation of inhibitors, (iii) consuming less energy, (iv) using inexpensive and environmentally friendly chemicals, (v) being able to recover catalysts, and (vi) producing less residuals (Balan, 2014; Taherzadeh and Karimi, 2008). In addition, the selection of pretreatment technology must consider the biological process after the pretreatment (Zheng et al., 2014). For instance, sulfuric acid pretreatment is not recommended for biogas production since sulfur could inhibit methanogen. However, it could be applied for biohydrogen production, which does not involve methanogenesis step. In addition, chemical pretreatment is not suggested for sludge/biofertilizer production due to the risk of chemical residue in the sludge. It should also be noted that pretreatment on AD might give a different result with pretreatment that is used prior to other biological processes. 3. Pretreatment methods to overcome the challenges This section addresses several pretreatment methods and their main effect towards the challenges possessed by recalcitrant and inhibitor-containing feedstocks to enhance AD in biogas production. Pretreatment methods for lignocellulosic materials and toxic feedstocks to improve anaerobic digestion performance are summarized in Table 1 and Table 2, respectively. Bearing in mind that the improvement of AD after pretreatment is not only the result of overcoming a single challenge, but also the contribution from some changes after pretreatment. However, one needs to choose one method as the pretreatment process prior to AD. Therefore, it is strongly recommended to apply a pretreatment method that can overcome the major challenge caused by the feedstock used to get a significant impact. 3.1. Pretreatment to remove lignin Pretreatments to delignify lignin from lignocellulosic materials include organosolv, ionic liquid, alkaline, (alkaline) hydrogen peroxide, and biological processes. During pretreatment, lignin is either solubilized in intact form or degraded into its simple monomers. 5

3.1.1. Organosolv pretreatment Organosolv pretreatment is a method that can dissolve and extract lignin from lignocellulosic structures in a relatively pure and primarily unaltered form (e.g. Mesa et al., 2011). Hence, this process is suitable to be applied to lignocellulosic materials with high lignin content. The intact lignin is a valuable product that can be used e.g. to generate electricity, heat, and to produce lignin-based adhesive (Pan et al., 2005). Remaining solid (cellulose with a low lignin content) can then be processed to produce biogas. Organosolv solvents like ethanol, ethylene glycol, and butanol are typically used. Mancini et al. (2018) applied organosolv pretreatment on wheat straw using ethanol (50% (v/v)) at 180°C for 1 h. The organosolv pretreatment removed lignin fraction by 14%. Hemicellulose was also affected by the treatment in which 51% of hemicellulose portion was lost. The results showed that the methane yield of the pretreated wheat straw was 15% higher over the yield from the untreated straw. To decrease the process temperature and to enhance the effectiveness of organosolv pretreatment, often, an acid catalyst is added. In the pretreatment of wheat straw, sulfuric acid with 4.4 mmol/L and Lewis acids with 8 mmol/L were individually added to ethanol and used as the pretreatment agent (Constant et al., 2016). The acid-catalyzed pretreatment was conducted at 160°C for 2 h. After pretreatment, delignification up to 91 and 87% was obtained when sulfuric and Lewis acid were the catalysts, respectively. Hemicellulose was concomitantly removed by 86% with sulfuric acid-added pretreatment and in the range of 54-97% depending on the cations of Lewis acids used. The percentage of cellulose after treatment with sulfuric acid was 69%. After treatment with Lewis acids, the percentage of cellulose between 55-96% was obtained. The best improvement of methane yield by sulfuric-acid and Lewis acids-catalyzed organosolv gave a similar value, which was about 56%. Likewise, addition of sulfuric acid as a catalyst in isopropanol-based organosolv pretreatment had a positive impact on lignin removal from sunflower stalks prior to AD (Hesami et al., 2015). Lignin removal of 21% was achieved after pretreatment at 160°C for 30 min with 1% H 2SO4. This 6

result was much higher compared with the treatment without catalyst, which was only 4.5%. However, it is worth to be mentioned that excess sulfate can activate sulfate reducing bacteria, which negatively affect methane production during AD due to nutrient competitions with methanogen (Yang et al., 2015). The higher lignin removal after the treatment at 160°C than the treatment at 180°C indicated that the presence of catalyst made milder conditions were more favorable in this case. The enhanced lignin removal contributed to the higher methane yield of the pretreated stalk that was 124% than that of the untreated one. There are two types of organosolv pretreatments i.e. pretreatment with and without catalysts, which lead to two different mechanisms. The presence of acid catalyst in organosolv pretreatment enables further lignin degradation by breaking the β-ether linkage in lignin structure. Cleavage of β-aryl ether bonds is important for depolymerization of lignin since β-aryl ether bond accounts for 40 to 65% of the total linkages in lignins (Zhao et al., 2017). Meanwhile, in pretreatment without catalyst, organic solvent breaks α-aryl ether linkages in a lignin structural unit containing a free phenolic hydroxyl group in the para position. The solvent is also capable in breaking lignin-carbohydrate bonds by cleaving ether linkages between the carbohydrate and the α-carbon atoms of lignin sidechains (McDonough, 1992). It should be noted that removal of lignin needs to leave high cellulose content in the pretreated materials as to produce high methane yield. Organosolv pretreatment is influenced by process temperature and time as well as the solvent used. Tailoring those factors in order to produce lignin with high quality and quantity is then of importance. 3.1.2. Ionic liquid pretreatment Ionic liquid (IL) is a molten salt and is one of solvents that can dissolve cellulose. Other examples of cellulose solvent are N-methyl-morpholine-N-oxide (NMMO), LiCl/N,N-dimethylacetamide (LiCl/DMAc), aqueous NaOH solution, alkali/urea and NaOH/thiourea aqueous solutions, tetra butyl ammonium fluoride/dimethyl sulfoxide system, metal complex solutions, concentrated phosphoric acid, and molten inorganic salt hydrates (Cao et al., 2017; Li et al., 2017; Wang et al., 2016). The 7

process using these solvents is therefore referred to as cellulose-based pretreatment. ILs have different capabilities to selectively dissolve either or all lignocellulosic components lignin, cellulose, and hemicellulose. When both cellulose and lignin are dissolved in ILs, anti-solvent water is used to precipitate the cellulose (Wang et al., 2012). In this way, cellulose and lignin can be separated. Low temperature ILs include imidazolium, pyridinium, ammonium, and phosphonium-based cations, along with alkyl or allyl side chains with anions such as chloride, acetate, and phosphonate (Lee et al., 2009; Haykir et al., 2013; Heinze et al., 2005). A polar aprotic solvent such as dimethyl sulfoxide (DMSO) can be used as a co-solvent to reduce high viscosity of ILs. For example, 1-N-ethyl-, 1-Nbutyl- and 1-N-hexyl-3-methlyimidazolium chlorides ([C2mim]Cl, ([C4mim]Cl, and ([C6mim]Cl) were used as solvents and DMSO was used as a co-solvent in the pretreatment of water hyacinth, rice straw, mango leaves, and spruce (Gao et al., 2013). The pretreatment was performed at 100, 120, 140°C for 2 and 4 h. The pretreatment of the four lignocellulosic biomasses was capable in removing lignin in the range of 15.4-64.8% depending on the biomass type, pretreatment temperature, time, and alkyl length. Based on the lignin extraction, reduction of crystallinity, recovery efficiency, and biogas production, [C4mim]Cl was selected as the optimum solvent for AD of the four biomasses. The results showed that AD of the four pretreated biomasses with [C4 mim]Cl for 2 h at 120°C resulted in higher biogas yields which were 97.6, 40.8, 23.4, 57%, respectively for the pretreated water hyacinth, rice straw, mango leaves, and spruce than that of the untreated biomasses. Li and Xu (2017) studied the effect of imidazolium-based ionic liquids (ILs) on decreasing lignin content in grass and further enhancement in AD. The four imidazolium-based ILs, i.e. [Bmim]Cl, [Bmim]OAc, [Bmim]BF4, and [Bmim]PF6 were individually mixed with water with a ratio of 1:2 before being used as a solvent in the IL pretreatment of grass at 120°C for 2 h. After pretreatment, the percentages of lignin decreased from 12.67% to 3.69-10.24%. The decrease of lignin by different anions in the ILs varied with [Bmim]OAc being the most powerful one. Accordingly, the grass pretreated with [Bmim]OAc had the highest methane yield, which was around 25% higher than the untreated one. 8

The extraction of lignin occurred due to the disruption of inter-and intra-molecular hydrogen bonding in biomass. Ionic liquid interacts and further dissolves the aromatic moieties of lignin through ℼ-ℼ and n-ℼ interactions between aromatic rings and the cations of IL (Wang et al., 2017). The anions ILs determine the hydrogen bond basicity and the ability to dissolve and swell the biomass (Hou et al., 2013), for which reason the level of lignin removal was different in the pretreatment of grass using four imidazolium-based ionic liquids (Li and Xu, 2017). The effect of alkyl chain length both on ILs pretreatment and the subsequent AD has been studied (Gao et al., 2013). Alkyl length and the type of anions are equally important as temperature and time as the factors influencing ILs pretreatment. It is, however, noteworthy that some anions are toxic to microbes. Of four imidazolium-based ILs, [Bmim]PF6 had the highest level of toxicity owing to the more fluorine ions and the higher hydrophobicity (Li and Xu, 2017). With an increase of ILs recycling cycles, the effectiveness of ILs in affecting lignin extraction and AD was maintained (Gao et al., 2013) or only slightly decreased (Li and Xu, 2017). The results suggest that ILs have good recyclability, which is beneficial when applied in industrial applications. 3.1.3 Alkaline pretreatment Alkaline pretreatment is one of the effective methods in solubilizing lignin. For example, rice straw was pretreated with NaOH at concentrations of 2-8% at 35°C for 8 days (Dai et al., 2018). After NaOH pretreatment, the percentages of lignin and hemicellulose decreased. The effect of NaOH was more extensive on lignin than on hemicellulose in which the percentage of lignin content was reduced from 12.5% in the untreated straw to only 5.5% at the lowest in the NaOH-treated straw. The removal of lignin as well as hemicellulose helped in increasing biogas production for rice straw that had been pretreated at all NaOH concentrations. The maximum biogas yield obtained with 6% of NaOH was 157% higher compared to the biogas yield produced without pretretment. Salehian et al. (2013) used NaOH (8% w/w) to pretreat softwood pine at 0 and 100°C for 10, 30, and 60 min. NaOH treatment for 10 min at high temperature reduced the contents of lignin and hemicellulose, especially acid9

insoluble lignin. The results showed that the extent of the pretreatment effect at high temperature occurred in shorter times. On the contrary, NaOH treatment at low temperature had a greater impact in carbohydrate composition and lignin removal in longer retention times. The NaOH treatment at 0 and 100°C resulted in 118 and 181% improvement in the methane yield, respectively. The effect of NaOH on the decrease of lignocellulosic recalcitrant depends on the operating temperature and concentration of NaOH used. The main effect of NaOH treatment using low concentration at high temperature, which causes increased digestibility of biomass, is for removal of lignin (e.g. Taherzadeh and Jeihanipour, 2012). On the other hand, pretreatment using high concentration of NaOH at low temperature can break the intramolecular hydrogen bonds between cellulose chains, causing a decrease in crystallinity (e.g. Porro and Be, 2007). Mustafa et al. (2018) used Ca(OH)2 in the pretreatment of sugarcane bagasse (SCB) with concentrations of 1.7-11.9% of dry weight SCB for 4 h at room temperature. The pretreatment decreased lignin and hemicellulose contents in SCB, while the percentage of cellulose increased. The degradation of lignin was more than that of hemicellulose reaching up to 41%. Anaerobic digestion of the pretreated SCB with 8.5% Ca(OH)2 showed 33% higher methane yield than that of the untreated SCB. In addition to strong bases, e.g. NaOH and Ca(OH)2, a weak base such as ammonia can be used in alkali pretreatment. Yuan et al. (2015) treated corn stover using 2-4% ammonia of dry weight of corn stover at 30℃ for 5 days. After pretreatment, hemicellulose decreased significantly by 3.4-29.34% and lignin content was partially removed by 5-6.6%. A small decrease of cellulose from 38.80 to 32.01% was observed. Nevertheless, the biogas yield of the pretreated corn stover was 27% higher with respect to the result of the untreated one. In order to enhance the efficacy of ammonia pretreatment, Hashemi et al. (2019) mixed ammonia with ethanol. Ethanolic ammonia-water mixture containing 10% v/v ammonia with 5, 25, and 50% ethanol with a ratio of 1:1 (w/w) was used in the pretreatment of SCB at 50 and 70°C for 12 and 24 h. The pretreatment significantly eliminated lignin and partially removed hemicellulose. Lignin removal was in the range of 50.8-77.4% depending on 10

the pretreatment conditions. The highest lignin removal was obtained at 70°C for 24 h with ethanol concentration of 50%. Moreover, hemicellulose removal, represented by xylan content, varied in the range of 12.8-38.2% at different pretreatment conditions. In general, ethanolic ammonia pretreatment gave higher glucan recovery, xylan recovery, and lignin removal over the sole-ammonia pretreatment. As a result, AD of the whole slurry of ethanolic-ammonia pretreated SCB produced a higher methane yield by ca 22% than the sole-ammonia pretreated SCB at the same pretreatment conditions. The methane yield was also 183% higher compared with the untreated SCB. The effectiveness of alkali in reducing lignin content can be explained by the fact that the hydroxyl group (OH) in alkali such as NaOH and Ca(OH)2 is capable to break the ester and ether bonds between lignin and polysaccharides and to weaken the hydrogen bonds between cellulose and hemicellulose. This action results in the separation and partial degradation of cellulose, hemicellulose, as well as lignin (Xiao et al., 2001). Furthermore, acidic fractions such as carboxylic or phenolic groups ionized in alkaline solution can increase the solubility of individual fragments and can induce the swelling of cell wall, which might promote the solubilization of lignin (Gierer, 1985). Temperature and concentration of alkali are important factors in alkaline pretreatment. Salehian et al. (2013) showed that high temperature is more favorable than low temperature since high temperature has more degrading effects. Adequate and suitable concentration of alkali is necessary to avoid over degradation of hemicellulose and cellulose. Also, high accumulation of Ca2+ may lead to the precipitation of calcium salt, which could inhibit methanogenic archaea (Gu et al., 2015). 3.1.4. (Alkaline) hydrogen peroxide pretreatment From the chemicals in the pretreatment of lignocellulosic materials, hydrogen peroxide is used because of its strong oxidizing capability to remove lignin content. Song and Zhang (2015) treated wheat straw using H2O2 with concentrations of 1-4% at 25°C for 7 days. After pretreatment, lignin and hemicellulose contents were reduced by 5.4-21.9% and 12.5-45.2%, respectively at different conditions. At the same time, cellulose loss by 9.3-30.2% also occurred. The loss of lignocellulosic 11

components and the increase of soluble fraction after pretreatment led to an improved methane yield by 50% after pretreatment with H2O2 at 3%. In order to enhance delignification process, hydrogen peroxide is usually performed in alkali condition to pH of 11-12 using NaOH. Dahunsi et al. (2019) used hydrogen peroxide with concentrations of 5.50-8.50% (v/v) in alkali conditions to treat sorghum bicolar stalk at 28-34.5°C for 80.67-91.66 min. In the most suitable conditions, i.e. H2O2 concentration of 6.8% (v/v) at 28°C for 85 min, sorghum bicolar stalk lost its lignin and hemicellulose by 73 and 42%, respectively. Alkaline H2O2 pretreatment (AHP) successfully enhanced biogas production with a yield of 56% higher than the untreated sorghum bicolar stalk. Alkaline H2O2 pretreatment with H2O2 concentration of 4, 8, and 12% (v/v) was applied on corn stalk for different times (32.5, 60, 87.5 min) and at different temperatures (36, 47, 58°C) (Venturin et al., 2018). The pretreatment reduced lignin and hemicellulose contents by 71 and 19%, respectively after pretreatment with 12% (v/v) of H2O2, at 58°C for 87.5 min. As a result, an increase of biogas yield by 22% over the untreated corn stalk was obtained. A modification of alkaline H2O2 pretreatment method was done by adjusting the high pH to a neutral pH (Katukuri et al., 2017). Using this technique, different concentrations of H2O2 ranging from 0.2 to 1% were used to treat a C4 rhizomatous grass, Miscanthus floridulus at 25°C for 24 h. The SEM analysis showed that H2O2 pretreatment significantly degraded hemicellulose and lignin in Miscanthus floridulus, especially in the H2O2 concentration of 0.8 and 1.0%. Due to the much lower of total phenolic compounds after H2O2 pretreatment with concentration of 0.8%, the maximum improvement of methane yield by 49% was obtained when Miscanthus floridulus was treated with 0.8% than that of 1.0% of H2O2. AHP in combination with steam explosion has been examined on paper tube residuals by Teghammar et al. (2010). The best condition was obtained at 220oC using a mixture of 2% of NaOH and 2% of H2O2 for 10 min, which resulted in decrease of lignin from 23 to 16% and increase the methane yield by 107%.

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Under normal conditions, hydrogen peroxide is only able to react with aliphatic part of lignin, without any changes and the occurrence of phenolic compounds degradation (Sun et al., 2000). However, under alkaline conditions (close to pH of 11.5) at relatively high temperature, hydrogen peroxide capable to attack phenolic compounds. Tuning peroxide solution to alkaline using hydroxide ions (OH-) makes H2O2 dissociate, forming radical species, i.e. HO·, HOO·, and O2·. These compounds will then react selectively with lignin in the oxidation reaction mechanism, which leads to delignification (Díaz et al., 2014; Xiang and Lee, 2000). Lignin degradation products such as phenolic compounds are harmful to methanogens. Applying an optimum concentration of H2O2 to have minimum concentration of phenolic compounds for enhanced methane production is necessary. Meanwhile, detoxification with laccase treatment could be an alternative to deal with phenolic compounds (Schroyen et al., 2017). In a frame to reduce environmental impact, AHP is conducted at neutral pH (Katukuri et al., 2017). It is noteworthy, however, that pH value affects delignification process. Hence, the amount of lignin content determines the choice of process in order to get the extent of delignification that is required. 3.1.5. Biological pretreatment Delignification of lignocellulosic materials using biological pretreatment can be performed by fungi, microbial consortium, and enzymes, which are able to degrade lignin and hemicellulose (e.g. Zheng et al., 2014). In fungal pretreatment, white-rot fungi are very popular to be used due to its effectiveness in degrading lignin among other fungi such as brown-, white-, and soft-rot fungi (e.g. Lundquist et al., 1977). Kainthola et al. (2019) employed Pleurotus ostreatus (PO), Phanerochaete chrysosposrium (PC), and Ganoderma lucidum (GL) on the pretreatment of rice straw prior to AD. PO, PC, and GL are among white-rot fungi, which are able to selectively disintegrate lignin yet still maintain high cellulose content (e.g. Dashtban et al., 2010). Fungal pretreatment, which was carried out at 30°C for 5 weeks, was able to remove lignin by 18-36%. The ratio of cellulose to lignin of the pretreated rice straw was in the range of 3.3-4.8, being PC as the fungus that had higher selectivity in 13

degrading lignin over the other two fungi. A lower cellulose to lignin ratio given by PO and GL indicated a simultaneous degradation of cellulose along with lignin, resulted in lower cellulose recovery. After pretreatment, improvement in the methane yield was obtained, which was 1.64 - 2.22 fold higher than the untreated rice straw. In order to increase delignification process and to enhance methane production, fungal pretreatment is combined with another pretreatment method. A combination of fungal pretreatment with alkaline pretreatment was performed to treat willow sawdust before AD (Alexandropoulou et al., 2017). Firstly, willow sawdust with mycelia suspension of Leiotrametes menziesii and Abortiporus biennis was incubated at 27°C. After 30 days of incubation, the fungal pretreated sawdust was treated with NaOH solution (1% w/v) at 80°C for 24 h. Regardless a lower lignin removal by A. bennis over L. menziesii, a higher solid material recovery of pretreated willow sawdust made A. bennis more favorable. Fungal pretreatment with A. Bennis combined with alkaline pretreatment enhanced lignin removal by 37% compared to when willow sawdust was pretreated only by A. Bennis. As a result, the increase in lignin removal improved methane yield by 50.1 and 115% over the sole-fungal pretreated and untreated ones. Instead of using white-rot fungi for delignification, pretreatment using their pure ligninolytic enzymes such as manganese peroxidase (MnP), lignin peroxidase (LiP), versatile peroxidase (VP), and laccase can be performed. The processes can be performed either by having enzyme pretreatment prior to AD or enzymes ensiling biomass in AD process. Corn stover was enzymatically pretreated using laccase (2 U/g) only and laccase combined with MnP (5 U/g) and VP (1.5 U/g) at 30°C up to 24 h (Schroyen et al., 2014). As phenolic compounds are the main degradation product of lignin degradation (e.g. Wu et al., 2013), the release of phenolic compounds into the liquid fraction indicated the occurrence of lignin degradation by the enzymes. Although being known to be inhibitory to microorganisms (Palmqvist and Hahn-Hagerdal, 2000), the low concentration of phenolic compounds in the solid fraction of the pretreated corn stover did not impact negatively the methane production. In fact, and regardless the loss of some soluble materials in the form of COD content, treatment with laccase after 14

24 h and with peroxidases enzymes after 6 h contributed to an increase in methane yield by 25 and 17%, respectively. Similar result with respect to enhancement in methane production was obtained when after mulching, summer harvested switchgrass was pretreated using LiP (1 U/ml) and MnP (2 U/ml) for 8 h (Frigon et al., 2012). The pretreatment temperature was 22°C for LiP and it was 37°C for MnP. The methane yields of the mulched-LiP pretreated and the mulched-MnP switchgrasses were higher by 29 and 42%, respectively compared to the only mulched-pretreated switchgrass. Combination of LiP or MnP pretreatment with NaOH pretreatment further increased the improvement of methane yields with the corresponding values for LiP-alkali and MnP-alkali pretreatments were 62 and 90%, respectively. Lignin degradation by enzyme coupled with lignin solubilization by alkali could be the reason for the further enhancement of the methane yields. In addition to fungi and ligninolytic enzymes, microbial consortium can be used in biological pretreatment. Unlike fungal pretreatment that can selectively degrade lignin, microbial consortium contains several hydrolytic microorganisms that have ability to degrade cellulose and hemicellulose (e.g. Yuan et al., 2016). Microbial consortia are usually screened from nature in decayedlignocellulosic biomasses. For example, microbial consortium OEM1 originated from spent mushroom substrate was used to treat pulp and paper mill sludge (PPMS) in a concentration of 10% (v/w) inoculum (Lin et al., 2017). After pretreatment using active microbial consortium OEMI at 28°C for 9 days, lignin as well as hemicellulose were degraded by 22 and 45%, respectively. An observation of cellulose degradation at a lower rate implied that the microbial consortium OEM1 tended to degrade lignin than cellulose. The methane yield of the pretreated PPMS with the active OEM1 was 1.4-fold higher compared to when PPMS was pretreated using sterilized OEM1. Yuan et al. (2016) used a thermophilic microbial consortium (MC1) in the pretreatment to enhance AD of cotton stalk. Selected from compost heaps, the thermophilic microbial consortium was reported to have lignocellulosic degradation ability (Cui et al., 2002). The microbial pretreatment was conducted at 50°C for 14 days under microaerobic condition. The pretreatment at different substrate loading 15

rates resulted in the loss of cellulose, hemicellulose, and lignin in the range of 36-77%, 25-69%, 618%, respectively. The loss of cellulose and hemicellulose was associated with the production of soluble chemical oxygen demand (sCOD) and volatile organic products (VOPs), which were used in the subsequent AD. Depending on the substrate loading rate and the pretreatment time, the improved methane yields of the mixture of residual cotton stalk treated by MC1 and the hydrolysate after pretreatment were higher by 107-136% compared with that of the untreated stalk. In fungal pretreatment, white-rot fungi produce highly active oxidative ligninolytic enzymes that act in catalyzing reactions, resulting in lignin degradation. MnP catalyzes the oxidation of Mn2+ to Mn3+ that can diffuse into the lignified cell wall and break the lignin bonds; LiP catalyzes the oxidation of non-phenolic units; laccase catalyzes the oxidation of phenolic compounds and aromatic amines; and VP catalyzes phenolic and non-phenolic aromatic compounds as well as catalyzes the oxidation of Mn2+ (e.g. Isroi et al., 2011). Lignin degradation process produces some compounds such as phenolic compounds that are inhibitors for AD process. From the aforementioned discussion, it is shown that enzyme pretreatment produces inhibiting compounds at a low inhibition level, which otherwise implies that lignin is not completely broken down. In microbial pretreatment, no sterilization is needed, which gives a benefit in terms of energy demand. However, as microbial consortium contains several kinds of microbes, there is competition in which hydrolytic and fermentative microbes work simultaneously in producing and consuming soluble materials to be used in AD process. Hence, pretreatment time becomes critical. Mild operating conditions and no chemical added make biological pretreatment is a less energydemanding and more environmentally-benign process. However, the slow reaction rate of biological pretreatment remains a problem. For enzyme pretreatment, the obstacle for its large-scale application up to now lies in the enzyme price. In order to achieve an effective and cost-efficient enzyme pretreatment, several strategies could be done, i.e. optimization of enzyme activity, enzyme recycle,

16

development of genetically modified organisms that can produce high quality enzymes, and improvement enzyme quality by genetic engineering (Housein Koupaie et al., 2019). 3.2. Pretreatment to increase accessible surface area and to decrease crystallinity Other objectives of pretreatment process besides removing lignin are (i) to increase porosity to have a more accessible surface area for microbes and (ii) to decrease the crystallinity index. As pretreatment impacts accessible surface area and crystallinity simultaneously, these two effects are discussed as one part in this Section. 3.2.1 Comminution pretreatment Comminution is a mechanical pretreatment that reduces biomass size. This method can stand alone as a pretreatment process or it can be done to prepare biomass before being treated by other pretreatment processes. Comminution, or mechanical pretreatment in general, aims to increase surface area and to reduce cellulose crystallinity of recalcitrant biomass as to enhance its biodegradability towards the digesting microbes. The reduction in size caused by mechanical pretreatment decreases the stressed area, which results in higher local pressure, and makes it possible to destroy the biomass crystal structure. Hence, the crystalline structure is transformed into an amorphous form (Karinkanta et al., 2018). Milling with ball mill is a common method for this purpose. Using this method, Pengyu et al. (2017) treated Pennisetum hybrid under wet and dry conditions for 3-12 h. In wet milling, deionized water was added into the milling tank. It was reported that with increased treatment time, the pore volume, surface area, and average pore diameter of the Pennisetum hybrid tended to increase. The median diameters were in the range of ca 47-234 µm for the dry and 149-290 µm for the wet. These sizes were corresponded to the reduction by 25-84% and 7-52% than that of the untreated Pennisetum hybrid. The increased surface areas in dry and wet millings were higher in the ranges of 60-179% and 39-243%, respectively compared to that of the untreated Pennisetum hybrid. Meanwhile, the opposite result was obtained on the crystallinity index (CI). The CI gradually increased with an increase in treatment time due to the higher removal of the amorphous 17

parts. The results showed that AD of the milled pretreated hybrid had the highest methane yields after dry milling in 3 h and wet milling in 6 h, which were 41 and 24% higher, respectively compared to that of the un-milled hybrid. When Mattonai et al. (2018) applied ball-milling pretreatment on commercial celluloses, a different result of the effect of milling on crystallinity was obtained. After pretreatment for 30-120 min, the CIs decreased up to 73-93% in an asymptotic trend. Apparently, the milling was able to break the hydrogen bonds causing in reduction of the cellulose crystallinity. Furthermore, a decrease by 20% of the degree of polymerization was also observed and it showed a linear trend along with the treatment time. Other than milling, comminution can be accomplished by two different comminuting machines, i.e. a road plastic sweeping brush with a steel mesh conditioner (crimper) and a corrugated roller with coarse steel surface (Tsapekos et al., 2018). These two comminutors were used to enhance methane production from meadow grass. It was revealed that both comminutors were able to create structural damages on the meadow grass morphology and increased the biodegradable area. With respect to the particle size, the comminutor with a corrugated roller had a higher number of small particles than that of the plastic roller due to coarsener material employed. It is noteworthy that excessive destruction could result in low methane yields. Larger surface area and smaller particle size could cause acidification during AD process. This was evidenced by the higher concentration of VFAs in wet milling than that of in dry milling, for the case of study by Pengyu et al. (2017). Moreover, changes in microbial community was observed where AD of wetmilled Pennisetum hybrid had lower richness in bacterial community than those for AD of dry-milled Pennisetum hybrid (Pengyu et al., 2017). For these reasons, less enhancement of methane yield was obtained after wet milling than that of dry milling. 3.2.2 Steam explosion pretreatment Steam explosion is also regarded as autohydrolysis since the carbohydrate components in the biomass undergo depolymerization during the process. In this process, high pressurized steam penetrates the pores of biomass, followed by the sudden pressure drop that causes biomass disintegration during 18

explosion. Steam explosion was applied to treat rice straw at 162-240ºC with reaction times in the range of 12-30 min and pressure of 6.5-29 bars (Steinbach et al., 2019). At moderate severity of the pretreatment conditions, the pretreated straw showed more porous structures. The acid-insoluble lignin increased, which could be the result of the conversion of hemicellulose and partly cellulose to water soluble or volatile compounds. AD of the pretreated straw at this condition resulted in an increase of methane yield by 32% compared to the untreated straw. Li et al. (2016) treated Miscanthus lutarioriparius, a woody perennial grass, using steam explosion prior to AD. The injected steam reached a pressure of 0.5, 1.0, and 1.5 MPa with the corresponding temperatures of 153, 180, and 198ºC. The residence times were 3, 5, and 10 min, respectively. After pretreatment, glucan and lignin contents increased while xylan content decreased due to hemicellulose degradation. Based on the FTIR analysis, hemicellulose degradation was the result of acetyl group removal during steam explosion, which caused an increase of CI of the pretreated grass. On the other hand, steam explosion with a condition of 1.5 MPa and 198ºC produced a large surface area of the fibers. In turn, the methane yield of steam-exploded grass (1.5 MPa, 198ºC, and 3 min time) was 51% higher compared to the yield of the untreated one. Under optimum conditions, steam explosion is able to increase surface area, degrade hemicellulose, which eventually enhances methane yield of the pretreated biomass. Degradation of hemicellulose was found to correlate with the increase of the accessible surface area. Increase in condition severity had a positive impact on bioconversion to methane. However, harsher conditions resulted in a lower methane yield (e.g. Lizasoain et al., 2016). The steam provided during pretreatment and the generated of acetic acid from acetyl groups in the hemicellulose lead to the hydrolysis reaction, which produces simple sugars such as glucose and xylose. If the conditions permit (e.g. high temperature, longer residence time), some inhibitors such as furfural, HMF or phenol derivatives can be formed. Therefore, a compromise between these two tendencies have to be settled down during process optimization. The biomass disintegration during steam explosion occurs due to three combined effects 19

including heat from the steam, shear force generated from moisture expansion and auto-hydrolysis of glycosidic bonds by organic acids released during the process (Avellar and Glasser, 1998). The process occurs as follows: the thermal expansion opens up the biomass cell wall and induces the autohydrolysis by acetic acid released during the process, it is then followed by rapid pressure drop to atmospheric level, resulting in the explosion of plant structure by the expanding vapor (Eom et al., 2019). During this process, lignin is partially depolymerized into cinnamyl alcohol derivatives through the homolytic cleavage of the predominant β-O-4 ether and other acid labile-linkage (Tanahashi, 1990). 3.2.3. N-methyl-morpholine-N-oxide pretreatment N-methylmorpholine-N-oxide (NMMO or NMO) is an industrial solvent in the Lyocell process for the production of cellulose fibers (e.g. Adorjan et al., 2004). Pretreatment using NMMO is usually conducted between 90 and 120°C (Perepelkin, 2007) in 0.5-30 h (Jeihanipour et al., 2010). Several studies showed that NMMO is effective in increasing methane production by increasing accessible surface area of biomass and decreasing crystallinity (Table 1). Prior to AD, Purwandari et al., (2013) treated the oil palm empty fruit bunch (OPEFB) using 73, 79, and 85% (w/w) NMMO solution at 90 and 120°C for 1, 3, and 5 h. The pretreatment did not affect the composition of OPEFB significantly. Yet, cellulose part in the pretreated OPEFB had a reduction in its crystallinity degree and an increase in its amorphous phase up to 78%. Accordingly, enhancement of methane yield by 48% was obtained by AD of the pretreated OPEFB. Similar to the result of Purwandari et al. (2013), no major changes in the carbohydrate and lignin contents was obtained after NMMO pretreatment of wheat straw with 85% (w/w) NMMO solution at 120°C for 3 h (Mancini et al., 2018). Likewise, AD of NMMO pretreated wheat straw produced a higher methane yield of 11% over the untreated one. Enhancement of 46% in water retention value after NMMO pretreatment implied an increase in the porosity of the straw, which was likely caused by the decrease in the cellulose crystallinity. When barley straw and forest residues were pretreated with 85% (w/w) NMMO solution at 90°C for 7 and 30 h, improved 20

methane yields by 92 and 114% were obtained, respectively (Kabir et al., 2014). Simons’ Stain analysis revealed that an increase in accessible surface area of the pretreated barley straw and forest residues was achieved. Based on the FTIR analysis, crystallinity index of forest residues decreased from 1.90 for the untreated to 1.12 for the pretreated one. NMMO is an organic solvent that has a highly polar N-O bond (Wilson, 2013). Having high polarity of N-O bond, NMMO penetrates into the crystalline area of celluloses, breaks the hydrogen bond network of the cellulose, and forms new hydrogen bonds with the cellulose (Cuissinat and Navard, 2006). After dissolution in NMMO, cellulose can be regenerated by rapid precipitation using water as anti-solvent (Kuo and Lee, 2009). During regeneration, the crystalline structure of cellulose is modified, changing from crystalline form into amorphous form. In this way, NMMO only affects the crystallinity of cellulose without changing the composition of lignocelluloses. One of advantages of using NMMO as pretreatment agent is the possibility of recycling NMMO, which have a positive effect for the processing cost. Nevertheless, the efficiency of the pretreatment after NMMO recycle is highly dependent on the composition of lignocelluloses. Pretreatment with recycled NMMO showed the same performance as the fresh NMMO on barley straw. However, the performance of pretreatment with recycled NMMO on forest residue decreased due to the high amount of lignin and bark, which might have deactivated NMMO after several recycles (Kabir et al., 2014). NMMO pretreatment is an environmentally friendly process as NMMO is a non-toxic and fully biodegradable chemical (Rosenau et al., 2003). Furthermore, there is no need for chemical neutralization after NMMO pretreatment (Hall et al.,1999). Nevertheless, reports by some studies demonstrated that the residue of NMMO has a negative impact on AD. Addition of NMMO at a concentration of 2.5 g/L to inoculum resulted in only 15% of the expected biogas production (Purwandari et al., 2013). Similarly, no methane was produced in the presence of 1% NMMO (Kabir et al., 2013). In addition, NMMO is also suggested to have inhibited the hydrolysis stage by 12% at concentration of 0.5% (Jeihanipour et al., 2011). 21

3.2.4 Acid pretreatment In acid pretreatment, inorganic acids such as sulfuric acid (H 2SO4), hydrochloric acid (HCl), as well as organic acids such as acetic acid (CH3COOH), citric acid (C6H8O7), and oxalic acid (C2H2O4) can be used to improve methane yields. In order to suppress acid-reducing bacteria as well as for environmental and economic reasons, dilute acid (e.g. 1-5%) is more favorable over the concentrated acid (30-70%). Among acids, sulfuric acid (H2SO4) is the most commonly used in acid pretreatment due to its effectiveness. For example, dilute-sulfuric acid pretreatment with concentration of 1% (v/v) acid at 121°C for 10-120 min was carried out to treat wheat plant prior to biogas production (Taherdanak et al., 2016). After pretreatment for 120 min, the content of xylan in the hemicellulose was greatly reduced (84%), while lignin was only slightly removed (15%). The CI of the pretreated wheat plant was reduced by 23%. The surface layer of wheat plant was destroyed, and led to an increase in pore size. As a result, an increase in methane yield was obtained after pretreatment for 120 min, which was 15% higher compared to the untreated wheat plant. Pretreatment of rice straw used three organic acids, i.e. acetic, citric, and oxalic acids at concentrations of 5-15% (w/w) at temperature of 100-140°C for 30-60 minutes (Amnuaycheewa et al., 2016). Significant removal of lignin and degradation of hemicellulose occurred. The results of FTIR analysis suggested that the pretreatment succeeded to alter the crystalline cellulose to its amorphous form. The biogas production yields of the three organic acids-pretreated rice straw increased 519-533% than that of the untreated straw. The explanation behind the change in the composition and structure of lignocelluloses is that acid breaks covalently bonded acetyl groups that forms xylan backbone. In addition, acid cleaves the glycosidic linkages between xylose and arabinose units (Kamireddy et al., 2013). As a result, hemicellulose is solubilized and cellulose content is reduced. Frequently, dilute-acid is applied at high temperature (above 100°C). However, high temperature is energy demanding and in this condition, autohydrolysis could occur (e.g. Sarto et al., 2019) and some inhibitors such as carboxylic acids, 22

furans, and phenolic compounds could also be generated if the conditions allow. Autohydrolysis is avoided since it reduces the content of cellulose as the substrate in AD process. The inhibitors have been studied to be able to inhibit microbial growth or metabolic processes. Furthermore, the use of H2SO4 potentially leaves a sulfate residue in the pretreated biomass, which can form H 2S due to the reduction of sulfate during AD. The presence of H2S was studied to could have inhibited methane production (e.g. Zheng et al., 2014). At acidic conditions, precipitation of degraded lignin could occur, which then coagulate and condense onto the biomass surface (Yang and Wyman, 2004). It is thus very important to have optimum conditions in terms of temperature, acid concentration, and time as to avoid those problems. 3.3. Pretreatment to deal with natural inhibitors There are two ways of dealing with naturally occurring inhibitors in materials such as fruit wastes, i.e. by recovering the bioactive compounds for use as valuable products and by removing them or reducing their concentration to a safe level for digesting bacteria (Table 2). 3.3.1. Recovery of bioactive compounds Pretreatment methods to recover bioactive compounds in fruit wastes include steam distillation (e.g. Martín et al., 2018&2010), solid-liquid extraction (e.g. Wikandari et al., 2015; Ruiz et al., 2016), steam explosion (e.g. Forgács et al., 2012), and combination of (hydro)thermal and extraction processes (e.g. Trujillo-reyes et al., 2019). Apart from being known to have antimicrobial effects, DLimonene is a valuable compound that is used in several industries such as perfumery, chemicals, cosmetics, medical, and food flavors. Martín et al. (2018) and Martín et al. (2010) applied steam distillation to recover D-Limonene from orange peel wastes. The steam distillation process had water to peel ratio of 6:1 for 1 h. This technique was able to recover D-Limonene from the orange peel by 70%. Semi-continuous mode of AD of the pretreated peels showed a biodegradability, measured as COD, up to 96.7% (Martín et al., 2018) and 84-90% (Martín et al., 2010), respectively. According to the principle of steam distillation process, steam passes through the orange peels that contain orange 23

oil. Steam would condense and form a mixture of immiscible liquids; oil and water. The incoming steam evaporates this mixture and orange oil can be then separated. The presence of two immiscible fluids also makes steam distillation allow evaporation of temperature-sensitive essential oils at a lower temperature. This is beneficial as oil decomposition can be minimized. Ruiz et al. (2016) treated orange peel using solid-liquid extraction with ethanol as solvent. During solid-liquid extraction, D-Limonene is leached into the solvent, and the essential oil glandule is ruptured. In this way, D-Limonene can be released and later diffuses into the solvent (Negro et al., 2018). The ethanol extraction of orange peel was carried out with a peel to solvent ratio of 1:10 for 60 min at 40°C (Ruiz et al., 2016). Drying process was conducted to completely remove ethanol in the pretreated peel. Using this method, a nearly 100% of D-Limonene was extracted and an increase of 34% of the methane potential compared to the untreated peel was obtained. Recovery of DLimonene from orange peel using hexane as the solvent was also studied (Wikandari et al., 2015). The orange peel was either homogenized or chopped. The hexane extraction was performed at 20 and 40°C for 10 and 300 min with peel to solvent ratios of 1:12 and 1:2. Vacuum filtration and evaporation were applied to separate D-Limonene from hexane and as to remove or to recycle hexane residue in the peel. This technique produced D-Limonene recovery of 82% as the maximum, However, it did not gave high methane production due to the inhibition effect of hexane residue in the peel after evaporation (Wikandari et al., 2015). Furthermore, the toxicity of hexane was more apparent in the orange peel pretreated with homogenization. Apart from steam distillation and leaching, steam explosion is another effective method to recover D-Limonene. Forgács et al. (2012) applied steam explosion prior to AD of citrus waste. Steam explosion was performed at a temperature of 15°C and the steam was directly injected at 60 bars for 20 min. It was then followed by explosion at atmospheric pressure. The D-Limonene was recovered in the expansion tank and gave a decrease in limonene concentration of citrus waste by 94%. Batch digestion of pretreated citrus wastes produced methane with a higher yield of 426% compared with 24

that of the untreated citrus waste. Being capable to open up the complex structure of materials, steam explosion might have broken the oil glandules in citrus material during the process. When oil glandules break, D-Limonene is released. Other fruits that contain antimicrobial compounds are strawberries and raspberries (Dias et al., 2017). Trujillo-reyes et al. (2019) combined hydrothermal and adsorption-desorption processes to extract phenolic compounds from strawberry and raspberry extrudates prior to their biomethanization. In the hydrothermal pretreatment, the berry extrudates were heated by steam injection and the temperature was kept at 150°C for 60 min. In the adsorption-desorption process, phenols were recovered from liquid phase of the pretreated strawberry and raspberry extrudates with efficiency of 33 and 82%, respectively. The results showed that this combination of pretreatments enhanced AD of a mixture of the solid phase of the pretreated berries and the de-phenolized liquid phases. The methane yields were higher by 28 and 11% than their corresponding untreated strawberry and raspberry extrudates, respectively. Phenolic compounds are usually thermosensitive (Struck et al., 2016). Therefore, lowtemperature thermal pretreatment is a good process to treat olive mill solid waste (OMSW) (Serrano et al., 2017). This process allows the extraction of phenolic compounds with minimal degradation. Low-temperature thermal pretreatment was conducted at temperature of 65°C for 90 min. In the adsorption-desorption process, 13-96% of individual phenolic compounds could be extracted from the liquid phase of the pretreated OMSW. After extraction, the methane yields of the de-phenolized liquid phase and a mixture of the solid phase and the de-phenolized liquid phase were 24 and 22% higher, respectively, than that of the untreated OMSW. Bioactive compounds are products valuable for many industries. It is of importance that the pretreatment methods aiming at their recovery avoid degradation of these compounds. Depending on the initial concentration of bioactive compounds in fruit materials, some pretreatment methods can be considered. For example, solvent extraction, steam explosion, and combination thermal and extraction processes could be applied to extract low concentration of bioactive compounds, whereas 25

steam distillation is effective when the concentration of bioactive compounds is high. Furthermore, the price of solvent and energy consumption are important factors in determining the pretreatment method of choice. In terms of energy, solvent extraction is more favorable than steam distillation and steam explosion due to lower energy required (Ruiz et al., 2016) and lower process temperature (Wikandari et al., 2015; Forgács et al., 2012), respectively. For solvent recovery and to avoid its inhibition, the selection of solvent is a critical point in leaching. On top of that, all the aforementioned factors should be weighed and compared with the profitability obtained from the recovery of the bioactive compounds. 3.3.2. Removal of bioactive compounds In order to remove or to reduce the concentration of bioactive compounds in fruit wastes, aeration and biological pretreatments can be performed. Mizuki et al. (1990) applied aeration to remove peel oil from citrus peel prior AD. During pretreatment, the homogenized peels were incubated in an opentopped tank at 30ºC with air sparging at 10 L/(L peel.min). Removal of 99% of the peel oil after aeration for 12 h resulted in a stable or no inhibition of AD of the pretreated peel at a loading rate of 2 g/(L.day), which was not the case for the untreated peel. Aeration was also used to treat press liquors from citrus peel with air sparging of 2 L/min at 30ºC (Lane, 1983). Although aeration permitted the growth of limonene-degrading microorganisms in the liquor medium, the removal of citrus oil was due to evaporation and air stripping rather than degradation by microorganisms. The aeration of citrus peel press liquors for 8 h successfully removed 99% citrus oil. Despite the success of citrus oil removal, it should be noted that aeration potentially reduces COD value, and accordingly lower the methane yield in the AD process. Biological method either by fungi or enzymes is an alternative pretreatment process to remove bioactive compounds in fruit wastes. Ruiz et al. (2016) used Penicillium genus to treat orange peel (OP) at 25ºC for 1 week. In spite of 22% removal of D-Limonene from OP, no positive effect was observed on methane production and biodegradability of fungal pretreated OP. This was due to the 26

inhibition effect caused by α-terpineol, which was produced during the fungal pretreatment. Fungal pretreatment using Sporotrichum, Aspergillus, Fusarium, and Penicillium was also applied to treat orange processing waste (OPW) (Srilatha et al., 1995). As the fungi were originally isolated from the digested slurry of digester using fruit and vegetable wastes as well as cow-dung as the substrates, the fungi could already have had limonene-degrading capability. The results showed that the growth of fungi on the citrus waste at 30ºC for 96 h resulted in 55% removal of D-Limonene on a dry basis. The reduced D-Limonene content increased the loading rate of fungal-pretreated OPW by 2 folds in the subsequent solid-state AD compared with that of the untreated OPW. Combination of biological method using crude enzyme and stirring to remove D-Limonene was also applied (Akao et al., 1992). Maceration of citrus peel was performed by treatment with Aspergillus sp. crude enzyme at 30ºC for 48 h with continuous stirring. The soft texture of macerated peel made the release of citrus oil from its tissue by easier stirring. A removal of 95% peel oil by this combined technique increased the limiting load of the citrus peel by 50% in the AD process compared with when citrus peel was treated without enzyme. 4. Techno-economic aspects of pretreatments 4.1. Aspects influencing scale of application Due to limitations of e.g. high costs and/or complexity of the system, only some pretreatment methods for lignocellulosic feedstock end up at higher scale application of pilot- and full-scales. Physical pretreatments such as thermal hydrolysis and grinding are considered to be the most available approaches used in pilot- and full-scale implementation. While many studies in bench-scale of chemical pretreatments using alkali, acid, and peroxide are available (Carrere et al., 2016), these processes are limited for a larger scale. This is due to the chemical pretreatments that are relatively expensive, corrosive, and require additional treatment and/or solvent recovery. For instance, when using acid for a pretreatment process, a dose in the range of 0.5–1.0 g acid/g TSS is needed (Woodard and Wukasch, 1994), which is economically unattractive due to the large volumetric flowrate of waste. 27

In addition, acidification requires addition of alkali to control the pH in AD. A promising alternative is offered by biological pretreatments. This process is not energy-consuming, but it can be slower than physical and chemical pretreatments in case of processing time. 4.2. Pilot and full-scale applications Physical pretreatments available on a pilot-scale are e.g. thermal hydrolysis (Ferreira et al., 2013; Schieder et al., 2000) and grinding (Mönch-Tegeder et al., 2014; Silva et al., 2012). Types of feedstock used include kitchen garbage, wheat straw, and silages. All pretreatments have a positive effect on biogas production. To the best of our knowledge, there are no records of chemical and thermo-chemical pretreatments for lignocellulosic material in pilot and full-scale applications for AD. It is likely that these pretreatment methods are economically unattractive because of the high cost of chemicals and equipment. In addition, the pretreatments require a pH control sequence or contaminant removal, which adds to the complexity of the system. Biological pretreatments by composting using fungi and enzymes were used to treat the organic fraction of municipal solid waste and wheat straw, respectively in a pilot-scale application (Mata-Alvarez et al., 1993; Schimpf et al., 2013; Rouches et al., 2019). In agreement to bench- and pilot-scale tests, full-scale applications show a positive effect that significantly increases methane production. Full-scale applications can be built up by selfdesigned or using patented technology. For licensed technology, thermal hydrolysis has been commercially available for decades such as Cambi™ Thermal Hydrolysis Process (CambiTHP™ Cambi Group AS), Exelys/BioThelys™ (Veolia Waters Technologies), TurboTec ® (Sustec), LysoTherm® (ELIQUO STULZ), and Biorefinex (Biosphere Technologies) (Kor-Bicakci and Eskicioglu, 2019; Panther, 2017). Conclusions What can be learned from all the studies aforementioned is that, one should select the appropriate pretreatment method based on the target of interest depending on the characteristics of biomass. It is understood that individual effect of pretreatment does not contribute solely to the enhancement in 28

AD. Furthermore, it is important to choose a pretreatment method that does not generate pollutants to make the process more environmentally friendly and sustainable in the long term. The development of potential pretreatment methods is thus expected to improve not only the technical but also the economic feasibility of the process for industrial applications. Acknowledgement This work was financially supported by the Swedish Research Council. Electronic annex E-supplementary data of this work can be found in online version of the paper. References 1.

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Caption of Figure and Table: List of Figures Figure 1. The goal of pretreatments to overcome the challenges of biomass recalcitrant and presence of toxicants

List of Tables Table 1. Pretreatment methods of lignocellulosic materials for anaerobic digestion Table 2. Pretreatment methods for toxic feedstocks

Table 2. Pretreatment methods for toxic feedstocks

41

Pretreatment

Pretreatment

method

(substrate)

conditions Significant effect/chang

Results

after Referen

pretreatment

ce

es Recovery of bioactive compounds Steam distillation

Peel : water (w/w) = 1:6, 1h 70% of D- Biodegradability (chopped orange peel)

(Martín

Limonene

up to 96.7% and et

recovery

COD removal up 2018 & to 84-90%

Solid-liquid

Peel : ethanol (w/v) = 1:10, A

extraction

60 min, 40°C (orange peel)

al.,

2010)

nearly 34% improvement (Ruiz et

100% of D- in methane yield

al.,

Limonene

2016)

recovery Peel : hexane (w/v) = 1:2, 82% of D- No 40°C,

300

min Limonene

(homogenized orange peel)

recovery

increase

in (Wikand

methane

ari et al.,

production due to 2015) solvent inhibition

Steam explosion

15°C, 60 bar, 20 min (citrus 94% of D- 426% of methane (Forgács waste),

followed

by Limonene

pressure reduction to 1 atm Combination

yield improvement

recovery

et

al.,

2012)

of 150°C, 60 min (strawberry 33% and 82% Improved methane (Trujillo

hydrothermal and and raspberry extrudates)

of

phenolic yields by 28% (for -reyes et

compounds

strawberry

recovery

extrudates)

and 42

extraction

from

11%

(raspberry al.,

processes

strawberry

extrudates),

2019)

and raspberry respectively extrudates, respectively Combination

of 65°C, 90 min (olive mill 13-96%

low-temperature

individual

yields by 24% (for et

phenolic

the de-phenolized 2017)

extraction

compounds

liquid phase) and

processes

recovered

22% (for a mixture

thermal

solid waste )

of Improved methane (Serrano

and

from

al.,

the of the solid phase

liquid phase

and

the

de-

phenolized liquid phase), respectively Removal of bioactive compounds Aeration

30ºC, 2 L air/min, 8h (press 99% of citrus No inhibition in (Lane, liquors from citrus peel)

oil removal

the AD process in a 1983) long term

30ºC, 10 L air/L peel.min, 99% of peel No inhibition in (Mizuki 12h

(comminuted

and oil removal

homogenized citrus peel)

the AD process up et

al.,

to a loading load of 1990) 2 g/(L.day) for the pretreated 43

substrate.

The

process failed for the

unpretreated

substrate Fungal

25ºC, 1 week (orange peel)

22% of D- No

increase

in (Ruiz et

pretreatment with

Limonene

Penicillium

removal (dry production due to 2016) basis)

methane

inhibition

al.,

by

a

limonenederivative product Fungal

30ºC,

96h

(orange 55% of D- Increase loading

the (Srilatha

pretreatment using processing waste)

Limonene

rate

in et

al.,

Sporotrichum,

removal (dry solid-state AD by 2 1995)

Aspergillus,

basis)

folds

Fusarium, Penicillium Combination using enzyme

of 30ºC, 48h with continuous 95% of peel 50% increase in (Akao et

Aspergillus stirring (comminuted and oil removal and homogenized citrus peel)

limiting load for al., AD

1992)

stirring

Table 1. Pretreatment methods of lignocellulosic materials for anaerobic digestion

44

Pretreat Improved Pretreatment

ment

methods

conditio

Significant effects

methane

References

yield (%) ns Pretreatment to remove lignin Organosolv

Ethanol

14%

pretreatment

50%

51% of hemicellulose removal

(v/v)

of

lignin

removal

15

(Mancini et al., 2018)

at

180°C for

1h

(wheat straw) Ethanol

91% and 87% of lignin removal for

56 for both

(Constant

65% with H2SO4 and Lewis acid, respectively

H2SO4 and

et

4.4

Lewis acid

2016)

mmol/L

as the

of H2SO4 and

al.,

8

mmol/L

86%

and

97%

of

hemicellulose

catalysts

removal for H2SO4 and Lewis acid, respectively

of Lewis acid

as 69% and 55% of cellulose content for

the H2SO4 and Lewis acid, respectively catalysts at 160°C

45

for

2h

(wheat straw) Isopropa

21% of lignin removal

124

nol 50%

(Hesami et al., 2015)

at 160°C for

30

min with 1%

of

sulfuric acids

as

the catalyst (sunflow er stalks) Ionic

liquid [C4mim]

pretreatment

Cl

at

120°C for

water hyacinth:

(Gao et al., 2013)

97.6 2h

(water hyacinth, rice

15.4-64.8% of lignin removal

(biogas yield) rice straw: 40.8

straw,

mango

mango

leaves:

46

leaves,

23.4

spruce)

spruce: 57

Imidazoli A decrease in lignin content (12.67% in

(Li and Xu,

um-based the untreated to 3.69% in pretreated

2017)

ionic

grass)

liquids (ILs) 25 [Bmim]O Ac

at

120°C for

2h

(grass) Alkaline

NaOH

A decrease in lignin content (12.5% in

pretreatment

6% (w/w) the untreated to aprox.8% in the

(biogas

at

yield)

35°C pretreated straw)

157

(Dai et al., 2018)

for 8 days (rice straw) NaOH

A decrease in acid soluble lignin (AIL)

8% (w/w) content (29.5% in the untreated to

181

(Salehian et al., 2013)

at 100°C 23.6% in the pretreated pine) for

10

min

47

(softwoo d pine) Ca(OH)2 8.5%

39% of lignin degradation

33

at

(Mustafa et al., 2018)

room temperat ure for 4h (sugarcan e bagasse) 4%

A decrease in the contents of lignin,

ammonia

hemicellulose, cellulose

of

27 (biogas yield)

(Yuan

et

al., 2015)

dry

weight of corn stover with moisture content of 70% at 30℃ for 5

days

(corn stover)

48

Ethanolic

77.4%

of

lignin

removal

ammonia

28.4% of hemicellulose removal

183

(Hashemi et

-water

al.,

2019)

mixture containin g

10%

(v/v) ammonia with 50% ethanol (1:1 (w/w)) at 70℃ for 24h (sugarcan e bagasse) (Alkaline)

H2O2 3% 17.8%

hydrogen

at

peroxide

for 7 days 29.4% of cellulose removal

pretreatment

(wheat

25℃ 45.2%

of of

lignin hemicellulose

removal

50

removal

(Song and Zhang, 2015)

straw) H2O2

73%

of

lignin

6.8%

42% of hemicellulose removal

removal

56 (biogas yield)

(Dahunsi et al., 2019)

49

(v/v)

at

28℃ for 85

min

under alkali condition (sorghum bicolar stalk) H2O2

71%

of

lignin

removal

12%

19% of hemicellulose degradation

22 (biogas yield)

(v/v), pH

(Venturin et

al.,

2018)

11.5, 58°C for 87.5 min (corn stalk) H2O2

Significant removal of hemicellulose

0.8%

and lignin

(v/v)

49

(Katukuri, et al 2017)

at

25°C for 24h, pH neutral (Miscant

50

hus floridulus ) A

A decrease in lignin content (23% in

mixture

the untreated to 16% in the pretreated

107

ar et al.,

of NaOH paper tube residuals) 2%

(Teghamm

2010)

and

H2O2 2% at 220°C for

10

min with explosion (paper tube residuals) Biological

Phaneroc 36% of lignin removal

2.22 folds

(Kainthola

pretreatment

haete

et

chrysosp

2019)

al.,

osrium incubated at

30℃

for

5

weeks

51

(rice straw) Abortipo

54% of lignin removal

115

(Alexandro

rus

poulou

et

biennis

al., 2017)

incubated at

27℃

for

30

days followed by alkaline pretreatm ent using NaOH 1% (w/v) at

80℃

for

24h

(willow sawdust) Laccase 2 Lignin degradation U/g

at

30℃ up to

25

(Schroyen et

al.,

2014)

24h

52

(corn stover) LiP

1 Information not available

U/ml

at

LiPalkaline

22℃ and

(Frigon et al., 2012)

pretreatme

MnP

2

nt : 62

U/ml

at

MnP-

37℃ for

alkaline

8h

pretreatme

followed

nt : 90

with NaOH 7 g/L pretreatm ent (summer harvested switch grass) Microbia

22%

of

lignin

degradation

l

45% of hemicellulose degradation

1.4 folds

(Lin et al., 2017)

consortiu m OEM1 10%

53

(v/w) inoculum incubated at

28℃

for 9 days (pulp and paper mill sludge) Thermop

36

of

cellulose

removal

hilic

25%

of

hemicellulose

removal

136

(Yuan

et

al., 2016)

microbial 6% of lignin removal consortiu m (MC1) incubated at

50°C

for

14

days under micro aerobic condition , substrate concentra 54

tion 4%, ratio

of

substrate to inoculum 1:2 (cotton stalk) Pretreatment to increase accessible surface area and to decrease crystallinity Comminution

Dry

Dry milling: 60-179% of increase in

(Pengyu et

pretreatment

milling

surface

al., 2017)

for

area

3h Wet milling: 39-243% of increase in Dry

and

wet surface area milling: 41

milling Wet for

6h milling: 24

(Penniset um hybrid) Ball-

73-93%

milling

20% of decrease in polymerization

for

30- degree

120 min

of

decrease

in

CrI

(Mattonai et Data not

al.,

2018)

available

(commer cial

55

cellulose s) Steam explosion Steam pretreatment

A

much

more

porous

structure

(Steinbach

explosion (increase in surface area)

et

at 206ºC

2019)

for

30

min and pressure of

al.,

Increase in acid insoluble lignin due to 32 hemicellulose degradation

18.5

bar (rice Increase

in

water-extractable

straw) components Steam

Increase in surface area

(Li et al.,

explosion

2016)

at 198ºC 51

with pressure of

1.5

MPa for 3

min

(Miscant hus lutariorip arius)

56

N-methyl-

NMMO

morpholine-N-

85%

i

oxide

(w/w) at

2013)

pretreatment

120°C for

78% of crystallinity reduction

3h

(Purwandar et

al.,

48

(oil palm empty fruit bunch) NMMO

46% increase in water retention value

(Mancini et

85%

al., 2018)

(w/w) at 120°C for

11 3h

(wheat straw) NMMO

Increase in accessible surface area

(Kabir

85%

(barley straw and forest residue)

al., 2014) 92 (barley

(w/w)

straw)

solution at

90°C

for

et

7h

41% of decrease in CrI (forest residue)

114 (forest residue)

(barley straw)

57

and

for

30h (forest residue)

Acid

H2SO4

23% of decrease in CrI

pretreatment

1% (v/v)

(Taherdana k

84% of xylan degradation

at 121°C for

120

et

al.,

2016) 15% of lignin removal

15

min (wheat plant) Citric

Less crystalline cellulose structure

acid 12%

(Amnuayc heewa

Significant removal of lignin

(w/w) at

al., 2016) Degradation of hemicellulose

126°C

et

533 (biogas

for

60 yield)

minutes (rice straw)

CRediT author statement

58

Ria Millati: Conceptualization, Data curation, Roles/Writing - original draft, Writing- Reviewing and Editing. Rachma Wikandari: Conceptualization, Data curation, Roles/Writing - original draft. Teguh Ariyanto: Conceptualization, Data curation, Roles/Writing - original draft. Rinita Utami Putri: Data curation, Roles/Writing - original draft. Mohammad J. Taherzadeh: Conceptualization, Writing- Reviewing and Editing

Highlights    

Challenges in anaerobic digestion of lignocelluloses Dealing with hard-to-digest materials and feedstocks with natural inhibitors Selecting suitable pretreatment depending on the characteristics of the biomass Importance of technical and economic feasibility for large scale pretreatment

59